Methylmercury (MeHg) is a persistent environmental toxin that accumulates in fish and seafood. Understanding the risks of MeHg versus the benefits of essential nutrients in a fish diet is a priority human health issue. Toxicity of MeHg is dictate by its slow rate elimination rate, which is known to be highly variable from person to person. Methods to quantify MeHg elimination in people are grossly underdeveloped. Furthermore, the mechanisms of MeHg metabolism that are required for its elimination are not fully understood. Our objectives are two-fold: 1) to establish non-invasive methods to quantify the variable trait of MeHg metabolism and elimination in individual people consuming a normal fish diet and 2) to validate the putative role for intestinal bacterial enzymes in de-methylation of MeHg which facilitates its elimination. Our long-term goal is to use this method to investigate genetic and dietary factors that affect MeHg metabolism. We will work with a model first established in rodent models whereby de-methylation of MeHg by bacteria in the gut is a rate-limiting step in its elimination. However, the role of gut bacteria in MeHg de-methylation and elimination in humans remains to be characterized. A number of bacterial species in the human microbiome harbor Hg-detoxifying genes contained in the mer locus. Of particular importance to our hypothesis are the merB (organomercurial lyase) and merA (mercury reductase) genes, which carry out reductive de-methylation of MeHg. Our working hypotheses are that: 1) elimination of MeHg from the human body is dictated by the rate-limiting step of MeHg de-methylation and 2) the majority of MeHg de-methylation in humans occurs in the gut via bacterial mercury de-toxifying enzymes. With these hypotheses we predict differences in MeHg elimination rate between individuals will correspond with the abundance and activity of the merB and merA genes in the gut microbiome. In this study we will evaluate outcomes in human subjects fed meals of tuna with naturally occurring levels of MeHg. In Aim 1, we will determine the correlation of MeHg elimination rate with MeHg de-methylation status in human subjects subsequent to fish meal consumption. MeHg elimination rate will be determined by longitudinal Hg analysis of hair samples. MeHg de-methylation status will be determined as the percent Hgi in feces. In Aim 2, we will determine the influence of the abundance and activity of bacterial merB and merA on MeHg elimination rate and MeHg de-methylation status. We will investigate fecal samples of subjects receiving tuna meals for the abundance and diversity of merB and merA DNA and as well as the specific enzyme activity of merB and merA variants. Statistical correlations of merB and merA abundance and activity with MeHg elimination rate and with de-methylation status in feces will be determined. We anticipate the findings from this study will yield an invaluable method to identify individuals with a fast or slow MeHg metabolism, and furthermore resolve a potential mechanism for altering MeHg metabolism via changes to the gut bacterial composition.